Hydroisomerization treatment process for feeds from the fisher-tropsch process

ABSTRACT

The invention concerns a process for the hydroisomerisation treatment for feeds from the Fischer-Tropsch process. The catalyst is essentially constituted by 0.05% to 10% by weight of a precious metal and a silica (5-70%)/alumina support with a specific surface area of 100-500 m 2  /g. The catalyst has an average pore diameter of 1-12 nm, the pore volume of pores with diameters between the average diameter ±3 nm being more than 40% of the total pore volume. The dispersion of the precious metal is 20-100% and the distribution coefficient for the precious metal is greater than 0.1. The process is operated at 200°-450° C. at a partial pressure of hydrogen of 2 to 25 MPa with a VVH of 0.1-10 h -1  and a hydrogen/feed volume ratio of 100-2000.

BACKGROUND OF THE INVENTION

The present invention concerns a hydroisomerisation treatment processfor feeds from a Fischer-Tropsch process to produce lubricating oils.

In the Fischer-Tropsch process, synthesis gas (CO+H₂) is catalyticallytransformed into oxygenated products and essentially linear hydrocarbonsin the gaseous, liquid or solid state. These products are generally freeof heteroatomic impurities such as sulphur, nitrogen or metals. Theyalso contain practically no aromatics, naphthenes or, more generally,cyclic compounds. They can, however, contain a significant quantity ofoxygenated products which, expressed as the weight of oxygen, is lessthan about 5%, and less than 10% by weight of unsaturated compounds(generally olefinic compounds). These compounds cannot be used as theyare, in particular because of their cold behaviour properties which arenot compatible with the normal uses of petroleum cuts. The pour point ofa linear hydrocarbon containing 30 carbon atoms per molecule (boilingpoint about 450° C., i.e., part of the oil cut), for example, is about+67° C., while customs regulations require that a commercial oil musthave a pour point of less than -90° C. These hydrocarbons from theFischer-Tropsch process must, therefore, be transformed into morevalorisable products such as lubricating oils, after undergoingcatalytic hydroisomerisation reactions.

All the catalysts which are currently used for hydroconversion arebifunctional, combining an acidic function with a hydrogenatingfunction. The acidic function is provided by supports with large surfaceareas (generally 150 to 800 m² ·g¹) with surface acidity, such ashalogenated aluminas (in particular chlorinated or fluorinated),phosphorous-containing aluminas, combinations of boron oxides andaluminium, amorphous silica-aluminas and silica-aluminas. Thehydrogenating function is provided either by one or more metals fromgroup VIII of the periodic classification of the elements, such as iron,cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium andplatinum, or by combination of at least one metal from group VI such aschromium, molybdenum and tungsten with at least one metal from groupVIII.

The balance between the acidic and hydrogenating functions is afundamental parameter which governs the activity and selectivity of thecatalyst. A weak acidic function and a strong hydrogenating functionproduces catalysts which are less active and selective as regardsisomerisation while a strong acidic function and a weak hydrogenatingfunction produces very active and selective catalysts as regardscracking. A third possibility is to use a strong acidic function and astrong hydrogenating function to obtain a very active but also veryselective isomerisation catalyst. It is thus possible, by judiciouschoice of each of the functions, to adjust the activity/selectivitybalance of the catalyst.

SUMMARY OF THE INVENTION

Our research on a number of silica-aluminas has led to the discoverythat, surprisingly, the use of a catalyst containing neither halogen norzeolite and comprising a particular silica-alumina can produce catalystswhich are very active but also very selective as regards isomerisationof feeds as defined below.

More precisely, the catalyst of the invention is essentially constitutedby 0.05-10% by weight of at least one precious metal from group VIIIdeposited on an amorphous silica-alumina support which contains 5-70% byweight of silica and has a BET specific surface area of 100-500 m² /g,the catalyst having:

an average pore diameter of between 1-12 nm,

a pore volume of pores with diameters between the average diameter asdefined above reduced by 3 nm and the average diameter as defined aboveincreased by 3 nm, of more than 40% of the total pore volume,

a precious metal dispersion of between 20-100%,

a distribution coefficient for the precious metal of more than 0.1.

In more detail, these characteristics are:

Silica content: the support used to prepare the catalyst described inthis patent is composed of silica SiO₂ and alumina Al₂ O₃. The silicacontent, expressed as the percentage by weight, is between 5% and 70%,preferably between 20% and 60%, more preferably between 22% and 45%.This content can be accurately measured using X ray fluorescence. It isconstant over the whole of the catalyst, i.e., the silica concentrationis not higher at the catalyst surface, for example. The silica in thecatalyst is homogeneous.

Nature of precious metal: for this particular reaction type, themetallic function is provided by a precious metal from group VIII of theperiodic classification of the elements, in particular platinum.

Precious metal content: the precious metal content, expressed in weight% of metal with respect to the catalyst, is between 0.05 and 10,preferably between 0.1 and 5.

Precious metal dispersion: The dispersion, representing the fraction ofthe metal which is accessible to the reactant with respect to the totalquantity of metal in the catalyst, can be measured, for example, by H₂/O₂ titration. The metal is first reduced, i.e., it undergoes treatmentin a hydrogen stream at high temperature under conditions whichtransform all the platinum atoms accessible to hydrogen to the metal. Anoxygen stream is then passed under operating conditions which oxidiseall the reduced platinum atoms which are accessible to oxygen to PtO₂.By calculating the difference between the quantity of oxygen introducedand the quantity of oxygen leaving, the amount or oxygen consumed can bededuced. This value allows the quantity of platinum which is accessibleto oxygen to be deduced. The dispersion is thus equal to the ratio ofthe quantity of platinum which is accessible to oxygen over the totalquantity of platinum in the catalyst. In our case, the dispersion isbetween 20% and 100%, preferably between 30% and 100%.

Precious metal distribution: the distribution of the precious metalrepresents the distribution of the metal inside a grain of the catalyst,the metal being well or poorly dispersed. Thus it is possible to obtainpoorly distributed platinum (detected, for example, in a ring in whichthe thickness is substantially lower than the radius of the grain) butwhich is well dispersed, i.e., all the platinum atoms in the ring areaccessible to the reactants. In our case, the platinum distribution isgood, i.e., the platinum profile, measured using the Castaing microprobeanalysis method, has a distribution coefficient for more than 0.1,preferably more than 0.2.

BET surface area: the BET surface area of the support is between 100 m²/g and 500 m² /g, preferably between 250 m² /g and 450 m² /g, morepreferably between 310 m² /g and 450 m² /g.

Average pore diameter: the average pore diameter of the catalyst ismeasured from a pore distribution profile obtained using a mercuryporosimeter. The average pore diameter is defined as the diametercorresponding to the zero point of the curve derived from the mercuryporosity curve. The average pore diameter, as defined, is between 1 nm(1×10⁻⁹ meter) and 12 nm (12×10⁻⁹ meter), preferably between 2.5 nm(2.5×10⁻⁹ meter) and 11 nm (11×10⁻⁹ meter), more preferably between 4 nm(4×10⁻⁹ meter) and 10.5 nm (10.5×10⁻⁹ meter), and advantageously between3 and 9 nm.

Pore distribution: the catalyst of this patent has a pore distributionsuch that the pore volume of the pores with diameters between theaverage diameter as defined above reduced by 3 nm and the averagediameter as defined above increased by 3 nm (i.e., the average diameter±3 nm) is more than 40% of the total pore volume, preferably between 50%and 90% of the total pore volume, more advantageously between 50% and80% of the total pore volume and most advantageously between 50% and 70%of the total pore volume. The catalyst thus has a uniform poredistribution, more monomodal than bimodal.

Total pore volume of support: this is generally less than 1.0 ml/g,preferably between 0.3 and 0.9 ml/g, and more advantageously less than0.85 ml/g. In general, the support has a total pore volume of more than0.55 ml/g, preferably at least 0.6 ml/g.

The silica-alumina is prepared and formed using the usual methods whichare well known to the skilled person. Advantageously, the support iscalcined prior to impregnation of the metal, for example by heattreatment at 300°-750° C. (preferably 600° C.) for 0.25-10 hours(preferably 2 hours) in 2-30% by volume of steam (preferably 7.5%).

The metal salt is introduced using one of the usual methods fordepositing metal (preferably platinum) on a support surface. One of thepreferred methods is dry impregnation which consists in introducing themetal salt in a volume of solution which is equal to the pore volume ofthe catalyst mass to be impregnated. An acidic, neutral or basicsolution of a metal salt (in particular platinum) is suitable. Neutralsolutions (pH close to that of water) or basic solutions are preferred.Before reduction, the catalyst can be calcined, for example by treatmentin dry air at 300°-750° C. (preferably 520° C.) for 25-10 hours(preferably 2 hours).

Before its use in the hydroisomerisation reaction, the metal containedin the catalyst must be reduced. One preferred method for reducing themetal is treatment in hydrogen at a temperature of between 150° C. and650° C. at a total pressure of between 0.1 and 25 MPa. Reductionconsists, for example, of a 2 hour stage at 150° C. followed by raisingthe temperature to 450° C. at a rate of 1° C./min then a 2 hour stage at450° C.: during the whole of this reduction step, the hydrogen flow rateis 1000 l hydrogen/ l catalyst. It should also be noted that any ex-situreduction method is suitable.

The catalyst described is active, for example, for hydroisomerisation offeeds from the Fischer-Tropsch process, to obtain a large quantity ofproducts resulting from the hydroisomerisation of the paraffin moleculespresent in the initial feed. It is of particular interest to produceproducts which can then be used as components of lubricating products.

The feed is brought into contact with the hydroisomerisation catalyst ina hydroisomerisation zone (or reactor) at a partial pressure of hydrogenof 2 to 25 MPa, advantageously 2 to 20 MPa, preferably 2 to 18 MPa, at atemperature of 200°-450° C., advantageously 250°-450° C., preferably300°-450° C., and most preferably 320°-450° C., or 200°-400° C.,300°-400° C., or 320°-400° C., at an hourly space velocity of 0.1-10h⁻¹, advantageously 0.2-10 h⁻¹, preferably 0.5-5 h⁻¹, at a hydrogen/feedvolume ratio of 100 to 2000. The effluent from the hydroisomerisationreactor is fractionated into different conventional petroleum cuts suchas gas, petrols, middle distillates and "isomerised residue"; thefraction termed "isomerised residue" represents the heaviest fractionobtained during fractionation and the oily fraction is extracted fromthis fraction. The oily fraction is traditionally extracted during anoperation termed dewaxing. The choice of temperatures during thefractionation step for the effluents from the hydroisomerisation reactorcan be widely varied depending on the specific requirements of therefiner.

When the amounts of unsaturated or oxygenated products is likely tocause too great a deactivation of the catalytic system, before enteringthe hydroisomerisation zone the feed from the Fischer-Tropsch processmust be hydrotreated in a hydrotreatment zone. Hydrogen is reacted withthe feed in contact with a hydrotreatment catalyst whose role is toreduce the concentration of unsaturated hydrocarbon and oxygenatedmolecules produced during the Fischer-Tropsch process. The effluent fromthis hydrotreatmen zone is then treated in the hydroisomerisation zone.

The hydrotreatment catalyst is a non cracking catalyst comprising atleast one matrix, preferably alumina based, and at least one metal ormetal compound which has a hydro-dehydrogenating function. This matrixcan also contain silica-alumina, boron oxide, magnesia, zirconia,titanium oxide, clay or a combination of these oxides. Thehydro-dehydrogenating function is preferably provided by at least onemetal or metal compound from group VIII, in particular nickel or cobalt.A combination of at least one metal or metal compound from group VI (inparticular molybdenum or tungsten) and at least one metal or metalcompound from group VIII of the periodic classification of the elements(in particular cobalt or nickel) can be used. The hydro-dehydrogenatingcomponent can also be a precious metal (preferably platinum orpalladium), for example in a concentration of 0.01-5% by weight withrespect to the finished catalyst. The concentration of non preciousgroup VIII metal, when used, is 0.01-5% by weight with respect to thefinished catalyst.

This catalyst can advantageously contain phosphorous; in fact, thiscompound provides hydrotreatment catalysts with two advantages: ease ofpreparation, in particular during impregnation with solutions of nickeland molybdenum; and higher hydrogenation activity.

The total concentration of group VI and VIII metals, expressed as metaloxides, is between 5% and 40% by weight, preferably between 7% and 30%by weight, and the weight ratio of group VI metal(s) over group VIIImetal(s) is between 1.25 and 20, preferably between 2 and 10, expressedas the oxide. The concentration of phosphorous oxide P₂ O₅ is less than15% by weight, preferably less than 10% by weight.

A catalyst containing boron and phosphorous, as described in Europeanpatent EP-A-0 297 949, can be used. The sum of the quantities of boronand phosphorous, respectively expressed as the weight of boron trioxideand phosphorous pentoxide, is about 5% to 15% with respect to the weightof support, and the atomic ratio of boron to phosphorous is about 1:1 to2:1, and at least 40% of the total pore volume of the finished catalystis contained in pores with an average diameter of over 13 nanometers.Preferably, the quantity of group VI metal, such as molybdenum ortungsten, is such that the atomic ratio of phosphorous to group VIBmetal is about 0.5:1 to 1.5:1; the quantities of group VIB metal andgroup VIII metal, such as nickel or cobalt, are such that the atomicratio of group VIII metal to group VIB metal is about 0.3:1 to 0.7:1.The quantities of group VIB metal, expressed as the weight of metal withrespect to the weight of finished catalyst, is about 2% to 30% and thequantity of group VIII metal, expressed as the weight of metal withrespect to the weight of finished catalyst, is about 0.01% to 15%.

Preferred catalysts are NiMo on alumina, NiMo on alumina doped withboron and phosphorous and NiMo on silica-alumina. Advantageously, eta orgamma alumina is used.

In the hydrotreatment zone, the partial pressure of hydrogen is between0.5 and 25 MPa, advantageously 0.5-20 MPa, preferably between 2 and 18MPa, at a temperature of is 250°-400° C., preferably 300°-380° C. Underthese operating conditions, the cycle time of the catalytic system is atleast one year, preferably 2 years, and the catalyst deactivation, i.e,the increase in the temperature to which the catalytic system issubjected to maintain conversion, is less than 5° C./month, preferablyless than 2.5° C./month. Under these conditions, the concentration ofunsaturated and oxygenated molecules is reduced to less than 0.5%,generally to about 0.1%.

The oils obtained from the process of the invention have very goodproperties because of their highly paraffinic character. The viscosityindex (VI) of the oil obtained after dewaxing the 380⁺ cut inMEK/toluene solvent, for example, is greater than or equal to 130,preferably greater than 135, and the pour point is less than or equal to-12° C. The yield of oil with respect to residue depends on the totalconversion of the feed. In the case of the present invention, this yieldis between 5% and 100% by weight, preferably greater than 10% and moreadvantageously greater than 60%. In one advantageous embodiment, atleast a portion of the non oily fraction, obtained during the dewaxingstep of the isomerised residue, is recycled to the hydrotreatment zoneand/or the hydroisomerisation zone.

The following examples illustrate the features of the invention withoutin any way limiting its scope.

EXAMPLE 1

Preparation of Hydroisomerisation Catalyst in accordance with theinvention

The support was a silica-alumina in the form of extrudates. It contained29.1% by weight of silica SiO₂ and 70.9% by weight of alumina Al₂ O₃.Before addition of the precious metal, the silica-alumina had a surfacearea of 389 m² /g and an average pore diameter of 6.6 nm. The total porevolume of the support was 0.76 ml/g.

The corresponding catalyst was obtained after impregnation of theprecious metal into the support. The platinum salt Pt(NH₃)₄ Cl₂ wasdissolved in a volume of solution which corresponded to the total porevolume to be impregnated. The pH of the water was 6.31 and the pH of thesolution obtained was 6.07. The solid was then calcined for 2 hours indry air at 520° C. The platinum content was 0.60% by weight. Theplatinum dispersion was 60% and the distribution was uniform across thegrain. The catalyst had a pore volume of 0.75 ml/g, a BET surface areaof 332 m² /g and an average pore diameter of 6.5 nm. The pore volumecorresponding to pores with diameters between 3.5 nm and 9.5 nm was 0.44ml/g, i.e., 59% of the total pore volume.

The pore distribution of this catalyst was as follows:

    ______________________________________                                        Pore diameter < 6                                                                        nm     pore volume = 0.16 ml/g =                                                                      21% of total                               6-5        nm     0.36 ml/g =      48%                                        15-60      nm     0.06 ml/g =      8%                                         >60        nm     0.17 ml/g =      23%.                                       ______________________________________                                    

EXAMPLE 2

Evaluation of Catalyst during a test carried out underHydroisomerisation conditions.

The catalyst whose preparation was described in the above example wasused on a paraffin feed from the Fischer-Tropsch synthesis, underhydroisomerisation conditions. In order to be able to use thehydroisomerisation catalyst directly, the feed was first hydrotreatedand the oxygen content reduced to below 0.1% by weight. The maincharacteristics were as follows:

    ______________________________________                                        Initial boiling point  201° C.                                         10% point              258° C.                                         50% point              357° C.                                         90% point              493° C.                                         Cut point              592° C.                                         Pour point             +67° C.                                         Density (20/4)         0.799                                                  ______________________________________                                    

The catalytic test unit comprised a single fixed bed up-flow reactor,into which 80 ml of catalyst was introduced. The catalyst was thensubjected to a pure hydrogen atmosphere at a pressure of 7 MPa to reducethe platinum oxide to platinum metal and the feed was then injected. Thetotal pressure was 7 MPa, the hydrogen flow rate was 1000 liters ofgaseous hydrogen per liter of injected feed, the hourly space velocitywas 1 h⁻¹ and the reaction temperature was 370° C.

The table below summarises the results for the original feed and thefeed after the hydroisomerisation operation.

    ______________________________________                                                      Hydrotreated                                                                          Hydroisomerised                                                       feed    effluent                                                ______________________________________                                        Reaction temperature (°C.)                                                             /         370                                                 Density at 15° C.                                                                      0.799     0.779                                               wt % 390.sup.- /effluents                                                                     65        80                                                  wt % 390.sup.+ /effluents                                                                     35        20                                                  Quality of 390.sup.+ residue                                                  Dewaxing yield  5.5       41.5                                                Oil quality                                                                   VI (viscosity index)                                                                          155       142                                                 Cut distribution                                                              IBP-220         1.5       13.3                                                220-370         53.5      60.6                                                370.sup.-       55        73.9                                                370.sup.+       45        26.1                                                Net selectivity of 220.sup.-                                                                  0         21.5                                                Net conversion (%)                                                                            0         42                                                  ______________________________________                                    

It can clearly be seen that the unhydroisomerised feed had a very lowoil yield, while after the hydroisomerisation operation, the oil yieldwas very satisfactory and the oil recovered had a very high VI (VI=142)and a pour point of -21° C. In addition, calculations showed that thegross naphtha 220⁻ yield (defined as the products with a distillationtemperature of less than 220° C.) was low since it was 18% by weight fora gross conversion of 370⁻ of 73.9% by weight.

EXAMPLE 2 bis

The same catalyst was brought into contact with the same feed, under thesame conditions except that the temperature was raised to 375° C. Theresults are shown in the table below:

    ______________________________________                                                      Hydrotreated                                                                          Hydroisomerised                                                       feed    effluent                                                ______________________________________                                        Reaction temperature (°C.)                                                             /         375                                                 Density at 15° C.                                                                      0.799     0.773                                               wt % 390.sup.- /effluents                                                                     65        91.8                                                wt % 390.sup.+ /effluents                                                                     35        8.2                                                 Quality of 390.sup.+ residue                                                  Dewaxing yield  5.5       60.5                                                Oil quality                                                                   VI (viscosity index)                                                                          155       140                                                 Cut distribution                                                              IBP-220         1.5                                                           220-370         53.5                                                          370.sup.-       55                                                            370.sup.+       45                                                            Net selectivity of 220.sup.-                                                                  0                                                             Net conversion (%)                                                                            0         77                                                  ______________________________________                                    

EXAMPLE 3

Evaluation of Catalyst of Example 1, during tests carried out withoutrecycling and with recycling of the non oily fraction obtained afterdewaxing.

The catalyst whose preparation was described in Example 1 was used underhydroisomerisation conditions with a paraffin feed from theFischer-Tropsch synthesis described above.

The catalytic test unit was identical to that described in the previousexamples. In one case, the reaction was carried out without recyclingand in the other, with recycling of the non oily fraction obtained afterdewaxing of the residue fraction: this non oily fraction obtained afterdewaxing is normally known as "dewaxing cake". The operationalconditions were adjusted to provide the same yields of residue fraction(i.e., the 390⁺ fraction).

The table below shows the catalytic performances obtained with andwithout recycling of the "dewaxing cake".

    ______________________________________                                                    without recycling                                                                       with recycling                                          ______________________________________                                        wt % 390.sup.- /effluents                                                                   80          74.6                                                wt % 390.sup.+ /effluents                                                                   20          25.4                                                Dewaxing yield                                                                              41.5                                                            wt % oil/feed 8.3         12.4                                                Net conversion of 390.sup.-                                                                 42.9        43.2                                                ______________________________________                                    

In all the cases, the oils obtained had a viscosity index (VI) of morethan 140 and a pour point of less than -12° C.

We claim:
 1. A process for the treatment of feeds from a Fischer-Tropschprocess to obtain lubricating oils, characterised in that the feedhaving been optionally hydrotreated is hydroisomerised in ahydroisomerisation zone, the effluent obtained is fractionated to obtainan isomerised residue, said residue being dewaxed to obtain oil and anon oily fraction, and in that the hydroisomerisation zone is operatedat a temperature of 200°-450° C., at aq pressure of 2-25 MPa, with anhourly space velocity of 0.1-10 h⁻¹ and a hydrogen/hydrocarbon volumeratio of 100-2000, using a catalyst which consists essentially of adeposit consisting essentially of 0.05-100% by weight of at least onepreviously reduced precious metal from group VIII on an amorphoussilica-alumina support, the catalyst containing neither zeolite norhalogen, said support containing a constant content of 5-45% by weightof silica and having a BET specific surface area of 100-500 m² /g, saidcatalyst having a homogeneous content of silica so that the silicaconcentration is not higher at the catalyst surface and said catalysthaving an average pore diameter of 1-12 nm, the pore volume of poreswith diameters between the average diameter reduced by 3 nm and theaverage diameter increased by 3 nm being greater than 40% of the totalpore volume, the dispersion of the precious metal being between 20-100%,and the distribution coefficient of the precious metal in the catalystbeing greater than 0.1.
 2. A process according to claim 1, characterisedin that the feed, before hydroisomerisation, is hydrotreated in ahydrotreatment zone using a catalyst comprising alumina and at least onehydro-dehydrogenation component, the temperature being 250°-400° C. andthe pressure being 0.5-25 MPa.
 3. A process according to claim 1,characterised in that at least a portion of the non oily fractionobtained from the dewaxing step is recycled to the hydroisomerisationzone and/or to the hydrotreatment zone.
 4. A process according to claim1, characterised in that the precious metal in the hydroisomerisationcatalyst is platinum.
 5. A process according to claim 1, characterisedin that the silica content in the support of the hydroisomerisationcatalyst is at least 20% by weight.
 6. A process according to claim 4,characterised in that the silica content in the support of thehydroisomerisation catalyst is between 22% and 45% by weight.
 7. Aprocess according to claim 1, characterised in that the total porevolume in the support for the hydroisomerisation catalyst is less than1.0 ml/g.
 8. A process according to claim 6, characterised in that thehydroisomerisation catalyst has a total pore volume of at least 0.3 ml/gand less than 0.9 ml/g.
 9. A process according to claim 1, characterisedin that the hydroisomerisation catalyst has an average pore diameter ofbetween 2.5 and 11 nm.
 10. A process according to claim 8, characterisedin that the average pore diameter is between 4 and 10.5 nm.
 11. Aprocess according to claim 1, characterised in that thehydroisomerisation catalyst has a pore volume of pores with diametersbetween the average diameter reduced by 3 nm and the average diameterincreased by 3 nm of between 50% and 90% of the total pore volume.
 12. Aprocess according to claim 1, characterised in that thehydroisomerisation catalyst has a pore volume of pores with diametersbetween the average diameter reduced by 3 nm and the average diameterincreased by 3 nm of 50%-80% of the total pore volume.
 13. A processaccording to claim 10, characterised in that the hydroisomerisationcatalyst has a pore volume of pores with diameters between the averagediameter reduced by 3 nm and the average diameter increased by 3 nm of50%-70% of the total pore volume.
 14. A process according to claim 1,characterised in that the support for the hydroisomerisation catalysthas a specific surface area of between 250 and 450 m² /g.
 15. A processaccording claim 1, characterised in that the support for thehydroisomerisation catalyst has a specific surface area of between 310and 450 m² /g.
 16. A process according to claim 1, characterised in thatthe support for the hydroisomerisation catalyst is impregnated with aneutral or basic solution of a precious metal salt, and the resultantimpregnated metal salt is reduced in a reduction step to elementalmetal, and wherein the impregnated catalyst is optionally calcined priorto the reduction step, said reduction step occurring prior to thehydroisomerization step.
 17. A process according to claim 1,characterised in that the hydroisomerisation zone is operated at apressure of 2-18 MPa, and a temperature of 300°-450° C.
 18. A processaccording to claim 1, characterised in that the process is operated at atemperature of 320°-450° C.
 19. A process according to claim 2, in whichthe hydro-dehydrogenation component is a combination of at least onemetal or metal compound from group VIII and at least one metal or metalcompound from group VI of the periodic classification of the elements,the total concentration of metals from groups VI and VIII, expressed asthe metal oxides, being between 5% and 40% by weight and the ratio ofgroup VI metal oxides to group VIII metal oxides being between 1.25 and20 by weight.
 20. A process according to claim 2, in which thehydro-dehydrogenation component comprises a precious metal selected fromthe group consisting of platinum and palladium.
 21. A process accordingto claim 2, in which, for the hydrotreatment catalyst, the concentrationof group VIII metal, expressed as the weight with respect to thefinished catalyst, is between 0.01% and 5% in the case of a preciousmetal and between 0.01% and 15% in the case of a non precious metal. 22.A process according to claim 2, in which the hydro-dehydrogenationcomponent further comprises phosphorous in an amount, expressed as theweight of phosphorous oxide P₂ O₅, of less than 15% with respect to thefinished catalyst.
 23. A process according to claim 1, wherein saidprecious metal dispersion is 30-100% and said precious metaldistribution is above 0.2.
 24. A process according to claim 1, whereinsaid support prior to impregnation is calcined by heat treatment at300°-750° C. for 0.25-10 hours in 2-30% by volume of steam.